1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a laser printer, which transfers a toner image carried by an image bearing member to a recording material using an electrophotographic technique. In particular, the present invention relates to an image forming apparatus including a transfer belt which performs transfer to a recording material and conveyance thereof.
2. Description of the Related Art
In an electrophotographic apparatus in which a recording material is carried and conveyed by a transfer belt stretched by a plurality of rollers, the recording material on the transfer belt passes a transfer nip portion and is electrostatically attracted to the transfer belt.
If the stiffness of the recording material is weak, the recording material cannot be separated from the transfer belt by merely using the curvature of a separation roller to stretch the transfer belt and the stiffness of the recording material. In other words, the recording material is kept stuck on the transfer belt at the position of the separation roller to cause a separation defect.
There has been a method for separating the recording material such that protrusions are uniformly formed on the separation roller stretching the transfer belt to provide a corrugation for the transfer belt at a separation position (refer to Japanese Patent Application Laid-Open No. 9-015987). The use of such a configuration allows the corrugation to be formed on the transfer belt at the separation position, however, great tension always locally act on the transfer belt. This causes local wear on the transfer belt, thereby irregularity in resistance affects transferability.
Japanese Patent Application Laid-Open No. 5-119636 discusses a method for decreasing wear due to deformation while deforming the sheet carrying the recording material to separate the recording material. Japanese Patent Application Laid-Open No. 5-119636 discusses a configuration in which a roller is provided as a lifting unit which can move to the positions where the transfer sheet is lifted from the inside and not lifted. In the method discussed in Japanese Patent Application Laid-Open No. 5-119636, the roller lifts the transfer sheet to separate the recording material. The transfer sheet is not lifted while the recording material is not separated.
If such a configuration is applied to the transfer belt, a lifting unit in which the roller can locally lift the transfer belt in a separating process is arranged on the downstream side of a transfer member which transfers a toner image to the recording material on the transfer belt in the direction in which the recording material is conveyed. In a case where the stiffness of a recording material such as thin paper is weak, a corrugation is provided on the recording material such that the recording material is conveyed with the transfer belt locally lifted, which allows the increase of stiffness of the recording material in the separating process.
A long space between rollers in the width direction produces an area where a corrugation is hard to be provided for the belt surface between the rollers. On the other hand, a short space between rollers in the width direction raises the belt surface between the rollers and makes unevenness on the belt surface small.
Thus, it is desirable to inhibit the belt surface between the rollers from rising producing a small unevenness on the belt surface even if the space between the rollers in the width direction is decreased.
On the other hand, even if the space between the rollers in the width direction is decreased, it is found that a space between the rollers in the direction in which the recording material is conveyed is increased to inhibit the belt surface from rising between the rollers.